Soft tissue implants are used to reinforce or replace areas of the human body that have acquired defects. The inclusion of biomaterials, which can work either by creating a mechanical closure or by inducing scar formation, has improved the results obtained with soft tissue implants. However, implanting large amounts of synthetic material increases the rate of local wound complications such as seromas (30-50%), paraesthesia (10-20%), and restriction of mobility (25%) (see Klinge et al., Eur. J. Surg. 164: 951-960, 1998). Loss of mobility can occur, for example, when soft tissue implants are used in abdominal wall closures. Following implantation, current biomaterials with initially low bending stiffness may turn into hard sheets that cannot be displaced to the same extent as the abdominal wall (i.e., the sheets do not exhibit 25% strain under forces of 16 N/cm (see Junge et al., Hernia 5:113-118, 2001)). As a consequence, excessive scar tissue can form, which will decrease mobility in the abdominal wall. In addition, implants can cause inflammation and connective tissue formation. These events appear to be closely related to the amount of material implanted, the type of filament, and the proportion of pores, which define the surface or contact area between the foreign material and the recipient tissues. In particular, large amounts of polypropylene, especially that where the surface has been greatly enlarged by processing multifilaments, induce a strong inflammatory response (see Klosterhalfen et al., Biomaterials 19:2235-2246, 1998). Histological analysis of explanted biomaterials has revealed persistent inflammation at the interface, even after several years of implantation. The persistent foreign body reaction is independent of the inflammation time, but considerably affected by the type of biomaterial (see Welty et al., Hernia 5:142-147, 2001, and Klinge et al., Eur. J. Surg., 165:665-673, 1999). The persistence of this reaction at the biomaterial-tissue interface might cause severe problems, particularly in young patients, in whom the biomaterial is expected to hold for prolonged periods of time.
There are currently several known soft tissue implants. Bard Mesh™ is a non-absorbable implant that is made from monofilament polypropylene fibers using a knitting process (C. R. Bard, Inc., Cranston, R. I.; see also U.S. Pat. No. 3,054,406; U.S. Pat. No. 3,124,136; and Chu et al., J. Bio. Mat. Res. 19:903-916, 1985). Additional non-absorbable meshes are described in, for example, U.S. Pat. Nos. 2,671,444; 4,347,847; 4,452,245; 5,292,328; 5,569,273; 6,042,593; 6,090,116; 6,287,316 (this patent describes the mesh marketed as Prolene™); and U.S. Pat. No. 6,408,656.
The meshes described above are made using synthetic fiber technology. Different knit patterns impart unique mechanical properties to each configuration. The implant surface area ratio has also been calculated for prior art knit biomaterials. The following formulas were used to calculate the surface area ratio:
Vmat=Wmat/Dmat where Vmat is the material volume, Wmat is the material weight, and Dmat is the material density which is 0.904 g/cm3 for polypropylene;
Lfiber=Vmat/((Π)(Rfiber)2) where Rfiber is the radius of the fiber and Lfiber is the length of the fiber;
Asurface=(Π)(Dfiber)(Lfiber) where Asurface is the surface area of the fiber used to construct the material and Dfiber is the diameter of the fiber; and
Surface Area Ratio=Asurface/Farea where Farea is the area of the biomaterial fabric used to obtain Wmat.
WeightFiberSurfaceProductConstruction(g/cm2)Diameter (cm)Area RatioBard MeshMonofilament Knit0.00960.0172.52Trelex MeshMonofilament Knit0.01120.0172.85ProleneMonofilament Knit0.00960.0152.91Mesh
The Gore-Tex Soft Tissue Patch™ is another non-absorbable implant (W. L. Gore & Associates, Inc., Flagstaff, Ariz.; see also U.S. Pat. Nos. 3,953,566; 4,187,390; 5,641,566; and 5,645,915) made from expanded polytetrafluoroethylene (ePTFE). This product is microporous, having pores of approximately 20 microns in diameter. The porosity of the Gore-Tex material may, however, be insufficient to allow incorporation into surrounding tissues; a minimum pore size of approximately 60 microns may be required for fibrous or collagenous material to grow into the patch (Simmermacher et al., J. Am. Coll. Surg. 178:613-616, 1994). Methods to improve tissue ingrowth are described in U.S. Pat. Nos. 5,433,996 and 5,614,284, and a method of laminating a layer of mesh-type material to the ePTFE has also been described. In addition, U.S. Pat. No. 5,858,505 describes a macroscopically perforated ePTFE material with perforations having a minimum diameter of about 100 microns, and methods for producing high strength multiple component articles made from ePTFE are described in U.S. Pat. Nos. 4,385,093 and 4,478,655. Biomaterials made from ePTFE, however, do not have displacement elasticity properties that would prevent injury at the biomaterial-tissue junction. The ePTFE has a relatively low displacement elasticity, which prevents the biomaterial from extending when physiological force is applied.
Another type of implant, referred to as a “reinforcing plate” has been developed for treating damaged tissues (WO 01/80774). It contains a non-woven material based on polypropylene and forms a plate with small circular perforations (non-woven films may also be described in the art as “biaxially-oriented” films). The plate is preformed in a circular shape for treating damaged tissues of the abdominal wall.
Absorbable soft tissue implants are also known. For example, there are devices composed of polyglycolic acid and non-absorbable filaments (see U.S. Pat. No. 3,463,158; see also U.S. Pat. No. 4,520,821). Absorbable fibers can be used to create a knit mesh (see U.S. Pat. Nos. 4,633,873 and 4,838,884), and a warp knit mesh has been developed to prevent adhesions composed of regenerated cellulose (U.S. Pat. No. 5,002,551). A non-woven mesh made from biodegradable fibers has also been described (U.S. Pat. No. 6,045,908), as has a mesh having two layers that degrade at different rates (U.S. Pat. No. 6,319,264).
The thickness for the commercially available implants disclosed above is provided in the table below. As indicated, the thinnest material available has a thickness of 0.016 inches.
ThicknessMaterialCompanyCode No.(inches)Bard MeshC. R. Bard/Davol1126600.026Prolene MeshJ&J/EthiconPML0.020Gore-Tex Soft Tissue PatchW. L. Gore14150200100.039Gore-Tex Soft Tissue PatchW. L. Gore13150200200.079ProLiteAtrium Medical1001212-000.019ProLite UltraAtrium Medical307210.016
Each of the implants presently in use has one or more deficiencies. For example, their construction can result in characteristics (e.g., wall thickness and surface area) that increase the risk of an inflammatory response or of infection; seromas can form postoperatively within the space between the prosthesis and the host tissues; due to material content, width, and wall thickness, surgeons must make large incisions for implantation (the present implants can be difficult to deploy in less invasive surgical methods); rough implant surfaces can irritate tissues and lead to the erosion of adjacent tissue structures; adhesions to the bowel can form when the implant comes in direct contact with the intestinal tract; where pore size is reduced, there can be inadequate tissue ingrowth and incorporation; and the pore size and configuration of the implants does not permit adequate visualization through the implant during laparoscopic procedures. Implants with increased thickness, surface area, and void area can lead to excessive scar tissue formation and implant encapsulation, which results in shrinkage and stiffness to the implant and surrounding tissue region. Accordingly, there remains a need for implants for repairing soft tissue and methods of making those implants.